Ticks are the most important vector of many infectious diseases in the United States. Lyme borreliosis and human granulocytic anaplasmosis are severe bacterial diseases that, in California, are vectored primarily by Ixodes pacificus ticks. Although pathogenic Rickettsia species have not been reported in I. pacificus, a novel Rickettsia species that is a close relative to endosymbionts in I. scapularis has been identified and phylogenetically classified in my lab. Most aspects of the biology and ecology of the Rickettsia species in I. pacificus remain poorly understood. In particular, a possible symbiotic association between the bacterium and its host has not been explored. My research pursues the following specific aims: i) to study routes of transmission of the Rickettsia species in I. pacificus, including transovarial and transstadial transmission routes that are involved in the overall spread of many endosymbionts;ii) to study localization and maintenance of the rickettsial infection in I. pacificus by real-time PCR and fluorescence in situ hybridization;and iii) to study the effects of the Rickettsia species on the fitness of I. pacificus by antibiotic curing. It is hoped that results from this project will contribute to better understanding the symbiotic nature of the Rickettsia species in I. pacificus and ultimately serve as a significant element in identifying novel molecular targets to block the bacterial transmission routes, and thus to suppress tick populations and better control tick-borne diseases in the long run. PUBLIC HEALTH RELEVANCE Ticks are second only to mosquitoes as vectors of human and animal diseases worldwide. In the United States tick-borne diseases affect more than 21,000 people annually (39, 100). Environmental and human health concerns related to the use of acaricides in the past have led to a search for alternative approaches to reducing tick-borne diseases. Since obligate intracellular bacterial symbionts are absolutely required by their hosts, disruption of bacterial symbioses decreases host fitness. My project seeks to understand the transmission and function of the Rickettsia species in Ixodes pacificus. My working hypothesis is that the Rickettsia species is a bacterial symbiont and has a symbiotic association with I. pacificus and contributes to the host's fitness. The long-term objective of the project is to identify molecular targets that are essential for the symbiotic relationship, such as bacterial genes that are essential to the transmission of the Rickettsia species in I. pacificus. Results from the project would be used to design novel control measures that could significantly block the transmission of the Rickettsia species in I. pacificus and decrease the economic losses and disease transmission by ticks.